1. Field of the Invention
The present invention relates to a method of making a photomask used in the fabricating of a semiconductor device.
2. Description of the Related Art
In general, the fabricating of a semiconductor device includes a photolithographic process in which a photoresist is deposited on a semiconductor substrate, and a pattern of a photomask (original layout) is transcribed onto the photoresist to form a photoresist pattern. The photomask is manufactured from a photomask blank comprising a transparent substrate, and a layer of opaque material disposed on the transparent substrate. The opaque material typically comprises chromium (Cr) and is etched to form an opaque pattern constituting the pattern f the photomask. Photomasks that have been widely used in recent years include phase shift masks and a half tone masks. A phase shift mask is one that includes a phase shift film disposed on the opaque layer, and the half tone mask is a mask wherein fine holes are formed in a layer that defines the mask pattern.
Once the mask pattern is copied onto the photoresist on the semiconductor substrate and the photoresist is patterned, the patterned photoresist is used as mask in a subsequent process of etching an underlying layer to form a component of the semiconductor device, such as a wiring layer. Accordingly, the dimensions of the patterned layer(s) of the photomask must be very precise to meet the ever-increasing demand for the scaling down of semiconductor devices.
A conventional method of making a phase shift mask will now be described below with reference to FIG. 1 through FIG. 3.
Referring to FIG. 1, a photoresist pattern 18 is formed on a mask blank in which an opaque film 18 is disposed on a transparent substrate 10. The opaque film 14 is patterned using the photoresist pattern 18 as an etching mask to form an opaque pattern 14a (see FIG. 2). In the case of a phase shift mask, the mask blank includes a phase shift film 12 disposed on the substrate 10 below the opaque film 18. On the other hand, a binary mask does not include the phase shift film 12. In the case of a half tone mask, fine holes are formed in the phase shift film 12 or the opaque film 14.
Referring to FIG. 2 and FIG. 3, a dimension L1 of the opaque pattern 14a is measured for checking the accuracy in the forming of the pattern 14a. Following the removal of the photoresist pattern 18, the phase shift film 12 is etched using the opaque pattern 14a as a mask to form a phase shift pattern 12a. 
In the conventional method of making a photomask as described above, a dimension of the opaque pattern is measured while the photoresist pattern is disposed thereon. If the measured dimension of the opaque pattern is smaller than a reference value, additional etching is carried out until the dimension of the opaque pattern is approximately the same as the reference value.
Unfortunately, in this conventional method a polymer 20 comprising carbon group is formed on the sidewall of the opaque pattern 14a. The build-up of polymer 20 thwarts the efforts for precisely measuring the actual dimension L2 of the opaque pattern 14a. Undoubtedly, the dimension of the opaque pattern 14a can be measured after the photoresist pattern 18 and the polymer 20 are removed. However, it then becomes substantially impossible to correct the opaque pattern 14a in the event that the dimension of the opaque pattern 14a is different from the reference value. Therefore, it is in fact necessary to measure the dimension of the opaque pattern 14a while the photoresist pattern 18 is disposed on the opaque pattern.